ARIZONA COOP E R AT I V E
E TENSION
College of Agriculture and Life Sciences
AZ1131
January, 2009
Water Use in Vegetables—
DRY BULB ONIONS
Arizona Water
Series No.25
Edward C. Martin, Donald C. Slack, E. James Pegelow
Introduction
Onion production in Arizona has remained relatively
stable since 2001, varying from 1000-2000 planted acres,
with an average yield of 487 cwt./acre. Market prices have
fluctuated greatly in the past four years from $8.80 per cwt.
in 2004 to $12.00 per cwt. in 2007. Onions are grown in
Maricopa, La Paz, Pinal, Yuma and Cochise counties.
Commercial Irrigation Management
Most dry onions in the state are irrigated using furrow
irrigation. There are some fields utilizing drip irrigation,
but economics make it difficult for most growers to switch.
However, it is quite common for growers to use sprinklers
for germination. The use of sprinklers help to reduce the
amount of water needed and reduces the potential for
salts to enter the seed row and cause emergence problems.
Onions are sensitive to salts; therefore, an ECe of 1.2 or less
should be used for irrigation.
Onions should be kept stress free throughout the growing
cycle. However, the most critical period for onions is
during bulb swelling. An irrigation threshold of 40% soil
water deficit should be targeted to avoid water stress. If
using tensiometers or resistance blocks, -40 to -60 centibars
should be used to trigger irrigation. Set the tensiometer or
resistance block at a depth of 6 to 12 in. This will give an
accurate representation of the moisture within the plant’s
rootzone.
Water Use by Dry Onions
Onions are grown in the winter, with planting starting
sometime in September or October, and harvesting taking
place in March, April and May. Peak water use occurs
towards the end of the growing cycle when the stems begin
to swell and the onions mature. At this point, the water
is removed, the tops are knocked down and the onions
are allowed to “dry down” in the field. Dry onions use
approximately 20 in. total water for the season.
Two graphs illustrate water use by dry onions throughout
the growing season. Figure 1 shows the consumptive use of
dry onions as a function of Heat Units After Planting (HUAP).
The temperatures used to develop this curve were 43°F for
the lower threshold and 68°F for the upper threshold. The
heat units should be calculated using the sinusoidal approach
developed by Snyder (1985). Information on daily maximum
and minimum temperatures can be obtained from AZMET.
Total heat units required for dry onions is about 2300.
Figure 2 shows the average daily water use for dry bulb
onions in central Arizona. This information was developed
using crop water use information obtained in studies at the
Maricopa Agricultural Center and applying it to weather
data gathered at the Maricopa Agricultural Center AZMET
station (Martin et al., 2001).
When determining irrigation water needs, do not forget
to incorporate the system’s efficiency. For example, if the
irrigation system has an efficiency of 75% and the crop
requirement is 2 in., apply about 2.7 in. to account for the
system’s inefficiency (2 in./0.75).
Maximum water use by dry onions is about 0.2 in. of
water per day. This can be as high as 0.35 inches, depending
on planting date and location. This peak occurs late in
the season when the stalks begin to swell, just prior to
terminating irrigation. The rootzone calculation used for
water management in onions is a depth of one foot.
References
(Bulb size)
AZMET, Arizona Meteorological Network on the web at
http://ag.arizona.edu/azmet
Martin, E.C., A.S. de Oliveira, A.D. Folta, E.J. Pegelow and
D.C. Slack. 2001. Development and testing of a small
weighing lysimeter system to assess water use in shallow
rooted crops. Transactions of the ASAE. 44(1):71-78
Snyder, R.L. 1985. Hand calculating degree days. Agric.
Forest Meteorol., 35: 353-358
For additional information, contact your local Cooperative
Extension office.
Figure 1. Average daily water use as a function of Heat Units After Planting
(HUAP) for dry onions planted in mid-September.
Any products, services or organizations that are
mentioned, shown or indirectly implied in this publication
do not imply endorsement by The University of Arizona.
ARIZONA COOP E R AT I V E
E TENSION
THE UNIVERSITY OF ARIZONA
COLLEGE OF AGRICULTURE AND LIFE SCIENCES
The University of Arizona
College of Agriculture and Life Sciences
Tucson, Arizona 85721
Edward C. Martin, Ph.D.
Extension Irrigation Specialist
Donald C. Slack, Ph.D.
Department Head
E. James Pegelow, Ph. D.
Senior Research Specialist
Figure 2. Average daily water use for dry onions utilizing a calendar schedule
for a mid-September planting date.
Contact:
Edward C. Martin
edmartin@cals.arizona.edu
This information has been reviewed by University faculty.
cals.arizona.edu/pubs/water/az1131.pdf
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture,
James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona.
The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color,
religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.
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